Gram-negative facultative bacillus A. actinomycetemcomitans (Aa) is a major periodontal pathogen. The mechanism of Aa colonization was not fully understood. Aa has been observed to form tenacious biofilms on abiotic surfaces. This F33 application will test the hypothesis that LPS core mediates the adherence of Aa biofilms to abiotic surfaces. Specific Aim I will examine the genetics and the structure of Aa LPS core. A working model of the structure of Aa LPS and its biosynthesis pathway will be proposed. The LPS core synthesis genes will be mutated individually. The structure of the LPS of Aa wildtype and mutants will be determined. The results will be used to revise the proposed model until it is in agreement with the experimental data. Specific Aim II will identify the minimal LPS core structure required for the adherence and biofilm formation of Aa. Specific gene deletion LPS mutants of Aa will be constructed based on information obtained from Aim 1. The specific LPS core mutants will be tested for their adherence and, biofilm formation on abiotic surfaces. The long term goal of this research plan is to identify novel strategy to prevent colonization of Aa. Understanding the role of LPS in Aa biofilm formation may help fulfill this goal.